1. Technical Field
The invention is directed to a piston ring.
2. Related Art
Piston rings are provided with wear protection layers on their running surfaces and/or ring flanges, in order to achieve the required life span. High cylinder pressures, direct injection, exhaust gas recycling, and other design features in recent engine developments, as well as alternative cylinder materials make increasing demands on piston rings.
Wear resistant layers are applied by means of thermal spraying processes, plating processes or thin layer technologies, and where required, are further processed through heat treatments or diffusion processes. Normally, the layers are substantially homogenous, and are therefore applied in an unstructured form. The wear resistance is adjusted through the corresponding hardness of the material.
All forms of phenomena which indicate thermal overload on the surface of the piston ring are typically grouped under the concept of scorch marks.
From DE 199 31 829 A1 there is known a plated hard chrome layer, which possesses crevices, in which are embedded diamond particles with a size of 0.25 to 0.5 μm. In addition, further particles of hard materials consisting of tungsten carbide, chrome carbide, aluminium oxide, silicon carbide, silicon nitride, boron carbide or boron nitride can be embedded in the crevices.
When high temperatures occur, the diamond particles are converted into graphite, which then assumes the function of a lubricant, and therefore prevents the formation of scorch marks. Thus, this layer also possesses good emergency operating characteristics, in particular due to the conversion of diamond into graphite at temperatures of approx. 700° C. or higher.
In order to make further improvements in the scorch mark behaviour of piston rings, layers of materials have hitherto typically been used, which have very high melting points, and which consequently need very high temperatures in order to cause their thermal overload. A typical example for these is chrome nitride, which is applied by means of a PVD process, and has a decomposition temperature of approx. 2,000 K.
In order to improve resistance to scorch marks and wear resistance, there is proposed in DE 10 2004 028 486 A1 a coating of several individual layers, which consist alternately of chrome and chrome nitride. The chrome nitride layers may consist of CrN, Cr2N or mixtures thereof. In order to avoid abrupt transitions, the coating process is so controlled that the individual layers of chrome nitride each possess a border of Cr2N and a core of CrN. Each individual layer is at least 0.01 μm thick. The maximum thickness is 10 μm. The total thickness of the coating is given as 5 to 100 μm.
U.S. Pat. No. 5,549,086 discloses piston ring coatings in TiN and CrN.
German DE 10 2004 032 403 B3 describes piston rings, which possess a graded CrN coating over a chrome adhesion layer, such coating having a nitrogen content which increases towards the outside.
From JP 2005-060810 A there are known piston rings for combustion engines, which are provided with a multiple layer coating system, the individual layers of which possess the same metallic components, and which only vary in their nitrogen content. The layer thicknesses of the individual layers are given as <1 μm. The layers are applied by means of a PVD process, in particular an arc process.
However, the resistance to scorch marks of the known layers is not satisfactory.
Lamni et al. J. VrC. Technol. A23 (4), 2005 Page 593 ff describes the microstructures and the nano-hardness of layers consisting of the three-component material systems Zr—Al—N and Zr—Cr—N. The layers are applied by magnetron sputtering, and have a thickness of 1 μm. With respect to the three-component material system Zri1-xCrxN within the range of 0≦x≦0.48 no change in the nano-hardness was detected.
It is the purpose of the invention to disclose a piston ring with a wear-resistant coating, which possesses a high degree of wear resistance.